Fish feeding in the area appear to be totally unaffected, yet sharks detect the substance in even minute proportions.

In the controlled environment of lab tanks, sharks have responded to even 0.1 part per millionfor example, they would likely respond to 12 ounces of the chemical in an Olympic-size swimming pool.

The semiochemical is even strong enough to awaken lab sharks from tonic immobility, an induced, "death-feigning" state during which researchers can go so far as to perform surgery without arousing the animal.

Historic Challenge

Shark repellents have been in development for decadeswith only limited success. Researchers have tried (and continue to try) everything from chemicals and cages to audio signals and electric fields.

During World War II widespread ocean combat and casualties led to large numbers of human-shark interactions. The Navy issued a chemical repellent called Shark Chaser to protect sailors and airplane pilots.

In the mid-1970s, marine biologist Eugenie Clark tested a natural repellent from acidic protein secretions of the flounder-like Red Sea Moses sole. Gruber also worked on the project in Israel, Egypt, and Japan. However, effective, natural supplies of the secretion were limited and synthetic versions proved expensive and unstable.

In the early 1980s Cold War developments renewed the Navy's lapsed interest in repellents.

"It came to light that the submarine fleet was being challenged by sharks," Gruber recalled. "We had subs, as did the Soviets, cruising around the Atlantic listening for each other with towed sonar arrays, and from time to time they experienced what was called the 'million dollar bite.'

The costly chomp occurred when sharks bit, and damaged, trailing arrays or listening devices known as hydrophonesin these cases, big rubber tubes about 2 inches (5 centimeters) in diameter and half a mile (800 meters) long.

"The sharks were really biting into these things," Gruber recalled, noting that the problem spurred his first involvement in the development of chemical repellents.

In the 1990s the Natal Sharks Board of South Africa developed and patented electronic repellent technology employed by professional divers on their cages.

Australian-based SeaChange Technology currently markets the technology on their Shark Shield line of electronic repellent products for divers, swimmers, and surfers. While some hail the devices as effective, they lack the possible range of uses a semiochemical repellent could offer.

Fisherman's Friend?

Semiochemical repellent could find its way into everything from clothing to fishing tackle.

The substance could be a boon for longline commercial fishing operations like swordfish boatsand for the sharks that they inadvertently catch.

"To make longline fishing a little more selective, to reduce the horrific bycatch, which is sometimes three or five wasted sharks for each targeted speciesthat would be fantastic," Gruber said.

Though the product must be tested on more species, recreational applications may soon include incorporating the chemical into bathing suits, sunblock, and wet suits.

While the chance of attack will always be very small, those who spend time in the water may breathe a bit easier knowing that they are chemically less appealing to sharks.

Yet the biggest beneficiary may turn out to be the sharks themselves. Helping them avoid human encounters may be critically important to their survival.